Question:
Dogs are friendlier than cats since they would rather be alone.

Who is they referring to in this sentence?
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Answer:
cats


Question:
A motorboat turns the propeller pushing the water backwards, thus it moves forward.

Who is it referring to in this sentence?
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Answer:
A motorboat


Question:
Dogs are friendlier than cats since they enjoy the company of people.

Who is they referring to in this sentence?
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Answer:
Dogs


Question:
The professor told John that he passed the exam.

Who is he referring to in this sentence?
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Answer:

John